The present invention relates to resinous compositions useful as dielectrics and more particularly to the combination of a thermoplastic film and reinforced hollow glass microsphere reinforced laminates for production of thin, low dielectric constant substrates.
A number of polyphenylene ether/polyepoxide compositions have favorable dielectric properties, and supposedly being useful in circuit board manufacture, are known. However, for the most part, these have not attained wide commercial use because of deficiencies in one or more properties. Thus, while the polyphenylene ethers are excellent dielectrics and the properties of combinations thereof with polyepoxides are favorable in this respect, they lack solvent resistance which is required in order for the circuit board to survive cleaning. Other deficiencies are found in areas such as flammability, solderability, and resistance to high temperatures. Moreover, times required for curing such compositions typically are too long for effective manufacture of circuit boards in large volume.
In addition to excellent dielectric properties, resinous compositions to be useful for printed circuit board manufacture should be highly flame-retardant. A V-1 rating, as determined by Underwriters Laboratories Test Procedure UL-94, is universally required with V-0 usually being necessary. The V-0 rating requires a flame-out time (FOT) of not more than 10 seconds in any trial and a cumulative FOT of not more than 50 seconds for 5 samples. As a practical matter, a maximum cumulative FOT of 35 seconds often is mandated by purchasers.
The fabricated board should not lose substantial weight and its surface should not be appreciably marred by contact with methylene chloride, a solvent commonly used for cleaning. Since conductive connections with a printed circuit typically are made by soldering, the board must be solder-resistant as evidenced by the lowest possible percent increase in thickness (Z-axis expansion) when exposed to liquid solder at 288.degree. C. In addition to all these properties of the cured material, a relatively short curing time is highly desirable.
In order to develop thin (less than 0.010 in. thick) laminates, dielectric constants and dissipation factors must be quite low for the laminate core for their use in printed wire board multi-layer constructions in order to minimize capacitance coupling between signal traces and ground planes, to minimize signal propagation impedances and delays, and cross-talk between adjacent signal traces. One method for lowering the dielectric constant and dissipation factor of laminate cores is by the incorporation of hollow glass microspheres (HGM) in the resinous system. For example, U.S. Pat. No. 4,661,301 proposes an HGM filled resin by a vertical extrusion double belt press arrangement. Japanese Publication 62-48710 (Application 60-187-917) proposes a formulation including polyphenylene oxide (PPO) linked resin, an initiator, and between 5 and 50% of silica or HBM. German Pat. No. 3,711,238 proposes fabric plies with 5-40% HGM content in an epoxy/hardener system. U.S. Pat. No. 4,238,641 proposes composite epoxy/HGM systems for hermetic R.F. connectors and coaxial cables. U.S. Pat. No. 4,141,055 proposes UV curable epoxy/HGM systems for cross-over structures. Finally, Haining and Herbaugh, in a 1979 IBM copyrighted article, propose to fill an epoxy resin system with 20-200 micron HGM particles (0.5-2 micron wall thickness) to lower the dielectric constant (E.sub.r) from 3.5 to less than 2.
With thin laminates (less than 0.010 in.), HGM filled thermoset formulations (greater than 10% by weight HGM content) tend to be quite brittle. Thus, there is a need to reduce the brittleness of such thin laminates, while providing improved dielectric properties and flame-retardancy.